I played through each of them (that is, until I got a Game Over), and came up with the following five most important commonalities:

1. A tower defense game involves enemies crossing from one side of the battlefield to another. Usually these enemies are very varied and some may take multiple hits to kill or may have special properties (for example, metal balloons in Bloons TD 4 cannot be hurt by ordinary shots but must be hit by something like a mortar).
   Bloons TD 4: balloons (bloons) follow the road from one part of the screen to the other, and the balloons range in type from simple red and blue balloons to metal, camo, and rainbow balloons.
   Warzone: there are various enemies that flow from the right side of the screen to the left, including fighter aircraft and tanks.
   Frontline: much like Bloons TD 4, they follow a road, and include various kinds of troops.
2. If they make it to their destination, that's not a good thing. In most cases, the player loses lives, health, or that kind of thing. Losing all one's lives/health results in a Game Over.
   Bloons TD 4: the player initially has 200 lives, but each balloon that makes it to the other end of the road subtracts one life.
   Warzone: the home base has a life meter, and when enemies deplete it by running into it, the game is over.
   Frontline: the player has 1,000 health, and when depleted, the game is over.
3. In order to prevent them from reaching their destination, the player is able to create "towers." These towers are not always actually towers, but may be something else, like monkeys (Bloons TD 4), automated gun platforms like machine guns (Warzone), or troops with shotguns and such (Frontline). Generally these so-called "towers" are automated and keep shooting at the enemy. They cost money to place. These "towers" are sometimes upgradable. Upgrades usually cost money. Upgrades make the towers cause more damage, make the towers fire faster, etc. Quite often, the games will explicitly state that it is better to upgrade a tower than to build more low-powered towers.
   Bloons TD 4: these are monkeys armed with things like darts, mortars, boomerangs, etc. and also things like spikes and spike platforms.
   Warzone: the player can build things like machine gun platforms, nukes, etc.
   Frontline: the player can build things like soldiers armed with standard guns, 12-gauge shotguns, etc.
4. There is generally a high scores option. All three browser-based tower defense games that I played had this option.
   Bloons TD 4: present
   Warzone: present
   Frontline: present
5. There is usually a difficulty option, as well (higher difficulties mean more enemies, a less circuitous path for the enemies, etc). And of course being able to pause the game is an essential feature as well; without being able to pause, the game would be too difficult.
   Bloons TD 4: there are a few axes of difficulty (in the easiest one, the difficulty takes more than 40 minutes to ramp up to truly difficult).
   Warzone: various initial configurations are present, with "default" being considered the best for novice players.
   Frontline: there are also multiple difficulty levels.

In conclusion, a "tower defense" game possesses these five main features. Though Wikipedia currently states the genre had its roots in Rampart, I think that earlier examples could be found, and the tower defense model can be found in many games not normally termed "tower defense." Examples include the strategic mini-game in Final Fantasy VII at Fort Condor, the play concept of certain levels in Final Fantasy XII: Revenant Wings, etc.